Manual and automatic probe calibration

ABSTRACT

Embodiments of the present disclosure include an optical probe capable of communicating identification information to a patient monitor in addition to signals indicative of intensities of light after attenuation by body tissue. The identification information may indicate operating wavelengths of light sources, indicate a type of probe, such as, for example, that the probe is an adult probe, a pediatric probe, a neonatal probe, a disposable probe, a reusable probe, or the like. The information could also be utilized for security purposes, such as, for example, to ensure that the probe is configured properly for the oximeter, to indicate that the probe is from an authorized supplier, or the like. In one preferred embodiment, coding resistors could be provided across the light sources to allow additional information about the probe to be coded without added leads. However, any device could be used without it being used in parallel.

PRIORITY CLAIM

This application claims priority benefit under 35 U.S.C. § 120 to and isa continuation of copending U.S. patent application Ser. No. 11/640,077,filed on Dec. 15, 2006, which is a continuation of U.S. patentapplication Ser. No. 10/757,279, filed on Jan. 13, 2004, now U.S. Pat.No. 7,496,391, which is a continuation of Ser. No. 10/005,711, filed onNov. 8, 2001, now U.S. Pat. No. 6,678,543, which is a continuation ofU.S. patent application Ser. No. 09/451,151, filed on Nov. 30, 1999, nowU.S. Pat. No. 6,397,091, which is a continuation of U.S. patentapplication Ser. No. 09/016,924, filed on Feb. 2, 1998, now U.S. Pat.No. 6,011,986, which is a continuation of U.S. patent application Ser.No. 08/478,493, filed on Jun. 7, 1995, now U.S. Pat. No. 5,758,644. Thepresent application incorporates the foregoing disclosures herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to more effective calibrationand use of light-emitting diodes. More particularly, the presentinvention relates to an apparatus and method of calibrating and usinglight-emitting diodes in a sensor for use with an oximeter system.

2. Description of the Related Art

Light-emitting diodes (LEDs) are used in many applications. In certainapplications, knowledge of the particular wavelength of operation of theLED is required to obtain accurate measurements. One such application isnoninvasive oximeters conventionally used to monitor arterial oxygensaturation.

In conventional oximetry procedures to determine arterial oxygensaturation, light energy is transmitted from LEDs, each having arespective wavelength, through human tissue carrying blood. Generally,the LEDs are part of a sensor attached to an oximeter system. In commonusage, the sensor is attached to a finger or an earlobe. The lightenergy, which is attenuated by the blood, is detected with aphotodetector and analyzed to determine the oxygen saturation.Additional constituents and characteristics of the blood, such as thesaturation of carboxyhemoglobin and scattering can be monitored byutilizing additional LEDs with additional wavelengths.

U.S. Pat. No. 4,653,498 to New, Jr., et al., discloses a pulse oximeterthat utilizes two LEDs to provide incident light energy of twodifferent, but carefully selected, wavelengths.

In conventional oximeters, the wavelength of each LED in a sensor mustbe precisely known in order to calculate accurately the oxygensaturation. However, the sensors are detachable from the oximeter systemto allow for replacement or disinfection.

When a sensor is replaced, the LEDs of the new sensor may have aslightly different wavelength for the predetermined LED drive currentdue to manufacturing tolerances. Accordingly, conventional oximetersprovide for indicating to the oximeter the particular wavelength of theLEDs for a given sensor. In one known system, a resistor is used to codeeach transmission LEDs. The resistor is selected to have a valueindicative of the wavelength of the LED. The oximeter reads the resistorvalue on the sensor and utilizes the value of the resistor to determinethe actual wavelength of the LEDs. This calibration procedure isdescribed in U.S. Pat. No. 4,621,643, assigned to Nellcor, Inc. Such aprior art sensor is depicted in FIG. 1.

SUMMARY OF THE INVENTION

In conventional oximeters which provide an indication of the operationalwavelength of each LED for each sensor, the oximeter systems areprogrammed to perform the desired calculations for various wavelengths.This complicates the design of the oximeter system, and therefore, addsexpense to the oximeter system. Accordingly, it would be advantageous toprovide sensors which exhibit the same wavelength characteristics fromsensor to sensor.

In addition, conventional sensors require an additional LED for eachadditional wavelength desired. For replaceable sensors, each LED can addsignificant total additional cost because of the large number of sensorsthat are used in hospitals and the like. Therefore, it would bedesirable to provide a sensor which provides more than one wavelengthfrom a single LED.

Many LEDs are observed to exhibit a wavelength shift in response to achange in drive current, drive voltage, temperature, or other tuningparameters such as light directed on the LED. The present inventioninvolves an improved method and apparatus to calibrate LEDs by utilizingthis wavelength shift. In addition, the present invention involvesutilizing the wavelength shift to allow a single LED to provide morethan one operating wavelength. The addition of a wavelength provides theability to monitor additional parameters in a medium under test withoutadding an LED. In oximetry, this allows monitoring of additionalconstituents in the blood without adding additional LEDs to the oximetersensor.

The present invention also involves an application of the wavelengthshift in LEDs to obtain physiological data regarding the oxygensaturation of blood without knowing the precise operational wavelengthof an LED in the sensor.

One aspect of the present invention provides a tuned light transmissionnetwork for transmitting light energy at a preselected wavelength. Thenetwork has a current source configured to provide a preselected sourcecurrent with a light emitting diode coupled to the current source. Thelight emitting diode is of the type that exhibits a shift in wavelengthwith a shift in a selected tuning parameter. Advantageously, the tuningparameter is drive current or drive voltage. A tuning resistor connectedin parallel with the light emitting diode has a value selected to drawat least a first portion of the preselected source current such that asecond portion of the preselected source current passes through thelight emitting diode. The second portion of the preselected sourcecurrent is selected to cause the light emitting diode to generate lightenergy of a preselected wavelength.

In the present embodiment, the tuned light transmission network alsocomprises a detector responsive to light energy from the light emittingdiode to generate an output signal indicative of the intensity of thelight energy.

Another aspect of the present invention involves a method forprecalibrating a light generating sensor. The method involves a numberof steps. A first level of current passing through a light source asrequired to operate the light source at a preselected wavelength isdetermined. A second level of current is then defined. The second levelof current is higher than the first level of current. The second levelof current forms a drive current. A resistor is then selected which whencoupled in parallel with the light source forms a tuned light sourcenetwork. The resistor is selected such that when it is connected inparallel with the light source, it draws a sufficient amount of thedrive current such that the first level of current passes through thelight source.

Another aspect of the present invention is a method of providing twowavelengths from a single light emitting diode. A light emitting diodeis selected of the type that exhibits a wavelength shift with a changein drive current through the light emitting diode for a range of drivecurrents. A source of electrical energy is coupled to the light emittingdiode to provide the drive currents. The light emitting diode is drivenwith a first level of drive current within the range of drive current tocause the light emitting diode to become active and operate at a firstwavelength in response to the first level of drive currents. The lightemitting diode is then driven with a second level of drive currentwithin the range of drive current and different from the first level ofdrive current to cause the light emitting diode to become active andoperate at a second wavelength in response to the second level of drivecurrent.

In an embodiment where the light emitting diode is configured totransmit light energy to a medium under test, the method comprisesfurther steps. While the light emitting diode is operating at the firstwavelength, light is transmitted as a first light energy at the firstwavelength through the medium under test. The first wavelength is chosenfor a first predetermined attenuation characteristic of the light energyas it propagates through the medium under test. The attenuated lightenergy is measured from the light emitting diode with a photodetector.In addition, while the light emitting diode is operating at the secondwavelength, light energy is transmitted at the second wavelength throughthe medium under test. The second wavelength is chosen for a secondpredetermined attenuation characteristic of the light energy as itpropagates through the medium under test. The attenuated light energy ismeasured at the second wavelength from the light emitting diode.

In one advantageous embodiment, the method is used to determine theoxygen saturation of blood, and the medium under test comprises aportion of the human body having flowing blood. In this embodiment, themethod further involves coupling the source of energy to a second lightemitting diode which operates at a third wavelength distinct from thefirst and the second wavelengths. Further, the change in wavelengthbetween the first and second wavelengths has a preselected value. Thirdlight energy is transmitted at the third wavelength through the mediumunder test, and the third light energy is measured after propagationthrough the medium under test. Based upon the measurements, the oxygensaturation of the blood is determined.

In one embodiment, parameters in addition to oxygen saturation may alsobe determined relating to the medium under test when the firstwavelength has a known value, and the change in wavelength between thefirst and the second wavelengths has a preselected value. In thisembodiment, value of the second wavelength is determined, and anotherparameter is calculated relating to the blood. In one embodiment, theanother parameter is the saturation of carboxyhemoglobin. Alternatively,another parameter is scattering. Yet another parameter isMethhemoglobin.

Advantageously, using the apparatus described above for tuning, thefirst light emitting diode is adjusted with an adjusting resistor suchthat the change in wavelength for an incremental change in currentmatches a preselected wavelength change. Preferably, adjusting involvesplacing the adjusting resistor in parallel with the first light emittingdiode, and selecting the value of the adjusting resistor to cause thefirst light emitting diode to exhibit the preselected change for theincremental change in current.

Yet a further aspect of the present invention provides an oximetersensor having a first light emitting device configured to generate alight at a first known wavelength with a resistor in parallel with thefirst light emitting device. Preferably, the light emitting devicecomprises a light emitting diode. In one embodiment, the resistorcomprises an encoding resistor having a value indicative of the firstknown wavelength value. The value of the encoding resistor issufficiently high such that the encoding resistor draws effectivelyinsignificant current during active operation of the first lightemitting device.

In another embodiment, the resistor comprises a security resistor,having a value indicative that the oximeter sensor is of a predeterminedtype. In addition, the value of the security resistor is sufficientlyhigh such that the security resistor draws effectively insignificantcurrent during active operation of the first light emitting device.

Still a further aspect of the present invention involves a method oftuning a light emitting diode to operate at a preselected wavelengthwithin a range of wavelengths the method involves selecting a lightemitting diode that exhibits a wavelength shift in response to a changein drive current within a range of drive current and driving the lightemitting diode with a first drive current. The wavelength of the lightemitting diode during operation at the first drive current is measured,and, if the light emitting diode is not operating at the preselectedwavelength, the drive current is adjusted within the range of drivecurrent to a second drive current such that the light emitting diodeoperates at the preselected wavelength.

Another aspect of the present invention involves a sensor configured totransmit and detect light. The sensor has at least one light emittingelement, the light emitting element having an emission with a centroidtransmission wavelength. The sensor further has first and secondphotodetectors, the emission of the light emitting element being withinthe response of the first and second photodetectors. A light directingmember is configured to direct light from the at least one lightemitting element to the first and second photodetectors. A filterpositioned between the second photodetector and the at least one lightemitting element has a transition band selected to encompass thecentroid transmission wavelength.

In one embodiment, the sensor comprises an oximeter sensor, and the atleast one light emitting element comprises first and second lightemitting diodes. Advantageously, the first light emitting diode has acentroid wavelength in the red range and the second light emitting diodehas a centroid wavelength in the infrared range. Advantageously, thefilter has a transition band which encompasses the centroid wavelengthof the first light emitting diode.

In one advantageous embodiment, the light directing member comprises anintegrating optical sphere having the first and second photodetectorspositioned about the sphere so as to receive substantially equivalentportions of light from the at least one light emitting element.

In another embodiment, light directing member comprises a beam splittingmember positioned to substantially equally divide light from the atleast one light emitting member and to direct substantially equalportions of the light to the first and the second photodetectors.

Still another aspect of the present invention involves a method ofdetermining the centroid wavelength of a light emitting element. Themethod involves providing a set of a plurality of predetermined ratios,each of the plurality of predetermined ratios corresponding to anassociated centroid wavelength. Light is transmitted from the lightemitting element to a first light detecting element to obtain a firstintensity, and light is transmitted from the light emitting elementthrough a filter which attenuates the light to a second light detectingelement to obtain a second intensity. A ratio of the second intensity tothe first intensity is then calculated. The ratio is compared to the setof predetermined ratios to reference the centroid wavelength of thelight emitting element.

In one embodiment, the first and second light detecting elementscomprise the same light detecting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a calibrated prior art oximeter probe;

FIG. 2 depicts a representational graph illustrating the relationshipbetween the extinction coefficients of three constituents of blood withrespect to the transmission wavelength of light transmitted through theblood;

FIGS. 3A and 3B depict exemplary LED characteristics;

FIG. 4A depicts a representation of a tuned oximeter sensor according toone aspect of the present invention;

FIG. 4B depicts an oximeter system with a digit for monitoring;

FIGS. 5A and 5B depict a representational diagram of one embodiment of aresistor for use in accordance with the present invention;

FIG. 6 depicts the averaging effect in the wavelength of twosimultaneously active LEDs with close transmission wavelengths;

FIG. 7 depicts an embodiment of an oximeter sensor according to anotheraspect of the present invention; and

FIGS. 8 and 8A depict exemplary embodiments of improved calibratedoximeter sensors;

FIGS. 9A and 9B depict alternative embodiments sensors in accordancewith of one aspect of the present invention relating to detecting thewavelength of light emitting diodes;

FIGS. 10A, 10B, 10C, and 10D depict graphs relating to the wavelengthdetection aspect of the present invention; and FIGS. 11 and 11A depictgraphs of filter response curves for various filters in accordance withthe wavelength detection aspect of the present invention.

FIGS. 12-15 depict four different probe configurations for use with thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention has applicability to the use of medical probes andLEDs in general. However, an understanding is facilitated with thefollowing description of the application of the principles of thepresent invention to oximetry.

The advantages of noninvasive techniques in monitoring the arterialoxygen (or other constituents) saturation of a patient are well-known.In oximetry, light of a known wavelength is transmitted through a medium(e.g., a human digit such as a finger) under test. The light energy ispartially absorbed and scattered by the constituents that make up themedium as the light propagates through the medium. The absorption andscattering of the light energy by any given constituent depends upon thewavelength of the light passing through the constituent, as well asseveral other parameters. The absorption by a constituent ischaracterized with what is known as the extinction coefficient.

FIG. 2 represents an exemplary graph 100 of the relationship between theextinction coefficient of three possible constituents of blood withrespect to the wavelength of light. Specifically, a first curve 102illustrates the relationship between the extinction coefficient ofoxyhemoglobin (oxygenated hemoglobin) with respect to the transmissionwavelength; a second curve 104 illustrates the relationship between theextinction coefficient of reduced hemoglobin with respect to thetransmission wavelength; and a third curve 106 illustrates therelationship between the extinction coefficient of carboxyhemoglobin(hemoglobin containing carbon monoxide) with respect to the transmissionwavelength. This relationship is well understood in the art.

One wavelength is required for each separate constituent in the medium.The wavelengths used for oximetry are chosen to maximize sensitivity ofthe measurement (i.e., oxygen saturation, etc.). These principles arewell understood in the art.

The amplitude of the energy incident on a homogeneous media having atleast one constituent under test is approximately related to theamplitude of the energy transmitted through the media as follows:

$\begin{matrix}{I = {I_{0}^{- {\sum\limits_{i = 1}^{N}{d_{i}ɛ_{i}c_{i}}}}}} & (1)\end{matrix}$

where I_(o) is the energy incident on the medium, I is the attenuatedsignal, d_(i) is the thickness of the i^(th) constituent through whichlight energy passes, E is the extinction (or absorption) coefficient ofthe i^(th) constituent through which the light energy passes (theoptical path length of the i^(th) constituent), and c_(i) is theconcentration of the i^(th) constituent in thickness d_(j). Aswell-understood in the art, this basic relationship is utilized toobtain oxygen saturation using conventional oximetry techniques.

It should be understood that the above equation is simplified fordiscussion purposes. Other factors such as multiple scattering alsocontribute to the resulting attenuation of the light energy. Multiplescattering is discussed in a paper by Joseph M. Schmitt entitled,“Simple Photon Diffusion Analysis of the Effects of Multiple Scatteringon Pulse Oximetry,” IEEE Transactions on Biomedical Engineering, vol.38, no. 12, December 1991.

However, for further discussion purposes, the simplified equation (1)will be utilized. In procedures based on oximetry technology, theaccuracy of the physiological measurement is impacted by the accuracy ofthe wavelength of the transmission LEDs because, as depicted in FIG. 2,the extinction coefficient is dependent upon the wavelength of thetransmission LED. In order to obtain oxygen saturation, two LEDs, one inthe red wavelength range and one in the infrared wavelength range, aretypically utilized in order to obtain the saturation measurement for apatient. Further, as set forth in Equation (1), the extinctioncoefficient is a critical variable in the equation. Accordingly, it isimportant that the oximeter be provided with information as to thespecific wavelength of the transmission LEDs for the sensor. However,the wavelength of different LEDs, although manufactured for a specifiedwavelength, varies, for the same drive current from LED to LED due tomanufacturing tolerances.

Wavelength Tuned LEDs

One aspect of the present invention provides an apparatus and method fortuning each LED in a sensor such that the operating wavelengths for LEDsdo not vary significantly from sensor to Sensor. The tuning is performedby utilizing the wavelength shift exhibited in many LEDs in response toa change in drive current. FIGS. 3A and 3B illustrate this wavelengthshift principle in two graphs. The graph 110 of FIG. 3A depicts (with acurve 112) current in the vertical axis versus voltage in the horizontalaxis for a typical LED. The graph 110 of FIG. 3A is well-understood inthe art. In the area referenced between the axis indicated A and B, justbeyond the shoulder of the curve 112, the wavelength of certain LEDsshifts in a substantially linear fashion in response to a correspondingchange in drive current or voltage. The amount of wavelength shift perincremental change in drive current typically differs for each LED(designed for the same wavelength), just as the operating wavelength forLEDs (designed for a specific wavelength) varies for the same drivecurrent from LED to LED.

FIG. 3B depicts an exemplary graph 120 of the wavelength of an LED inresponse to the drive current in the area of the shoulder depicted inFIG. 3A. This graph depicts in a curve 122 an exemplary wavelength shiftfor an LED in the red range in response to drive current changes. Theslope of the curve 122 depicted in FIG. 3B varies from LED to LED, asdoes the wavelength range. However, for conventional LEDs used in bloodoximetry, an incremental shift in drive current through the LEDs causessome incremental shift in the wavelength. Because this relationship issubstantially linear in the area just beyond the shoulder of the curve112 depicted in FIG. 3A, in one preferred embodiment, the shift isobtained in the area beyond the shoulder. The graph of FIG. 3B is notmeant to represent all LEDs, but merely to represent one possiblewavelength shift corresponding to a particular change in drive current.

Accordingly, one way to obtain a selected wavelength is to drive theLEDs with the current necessary to obtain the wavelength. However, suchembodiment would require an oximeter design which varies the LED drivecurrent for each sensor.

In one advantageous embodiment, in order to avoid the added complexityof oximeter system design, a resistor is placed in parallel with an LEDin order to adjust the drive current through the LED to a level whichwill result in a selected wavelength. In such embodiment, the oximetersystem is designed to operate at the selected wavelength for each LED inthe sensor. And, the oximeter need only provide a fixed drive current.Accordingly, in one embodiment, the design of the oximeter is simpler inthat it need not take into account variations of wavelength from sensorto sensor. The oximeter can simply be designed to operate at theselected wavelengths and have a fixed drive current.

Each LED sensor manufactured for the oximeter is tuned, using thewavelength shift, such that the LEDs in the sensor generate light at theselected wavelengths for the oximeter. FIG. 4 depicts one embodiment ofa tuned sensor 150, connected to an exemplary oximeter system 152,according to the LED tuning aspect of the present invention.

The sensor 150 is illustrated with a first light source 160 and a secondlight source 170, typically LEDs. A first tuning resistor 162 connectedin parallel with the first LED 160 forms a first tuned LED network 164.Similarly, a second tuning resistor 172 is connected in parallel withthe second LED 170 to form a second tuned LED network 174. The sensor150 further comprises a photodetector 180. A power source in theoximeter system, such as an LED driver 182, is coupled to the tuned LEDnetworks 164, 174 in order to provide a predetermined drive current atthe input of the tuned LED networks 164, 174. Advantageously, the LEDdriver 182 provides current to only one of the tuned LED networks 164,174 at any given time. The photodetector 180 is coupled to receiving andconditioning circuitry 184 in the oximeter system 152. In operation, thephotodetector receives the attenuated light energy and responds with anoutput signal representing the intensity of the alternative lightenergy. The oximeter system 152 further comprises a controller 190 withsupporting resources and a display 192. The oximeter system receives thesignals obtained from the sensor 150 and analyzes the signals todetermine information regarding the medium through which the lightenergy has been transmitted. It should be understood that the oximetersystem is depicted in simplified form for discussion purposes. Oximetersystems are well known in the art. One possible oximeter systemcomprises the oximeter system disclosed in pending U.S. patentapplication Ser. No. 08/320,154 filed Oct. 7, 1994, which has beenassigned to the assignee of the present application. Other oximetersystems are well known and can be designed to operate at the selectedwavelengths.

As depicted in FIG. 4B, for oximetry, a typical medium may include afinger 200 or an earlobe, as well-known in the art. Media such as thefinger and earlobe typically comprise a number of constituents such asskin, tissue, muscle, arterial blood and venous blood (having severalconstituents each), and fat. Each constituent absorbs and scatters lightenergy of a particular wavelength differently due to differentextinction coefficients. In general operation, the first LED 162 emitsincident light in response to the drive current from the LED driver 182.The light propagates through the medium under test. As the transmittedlight propagates through the medium, it is partially absorbed by themedium. The attenuated light emerging from the medium is received by thephotodetector 180. The photodetector 180 produces an electrical signalindicative of the intensity of the attenuated light energy incident onthe photodetector 180. This signal is provided to the oximeter system152, which analyzes the signal to determine the characteristics of aselected constituent of the medium through which the light energy haspassed.

The tuning is now explained with reference to the first LED 160. Thetuning is also applicable to the second LED 172. As explained above, inresponse to a particular drive current, different LEDs respond withdifferent wavelengths, even though the LEDs were manufactured togenerate the same wavelength. Tuning the first LED 160 in accordancewith the present invention involves determining the amount of currentrequired to operate the first LED 160 at the selected wavelength andadjusting the current through the first LED 160 in order to obtain theselected wavelength.

For instance, typical operational values for red LEDs used in oximetryrange between 645 nm and 670 nm. For a particular embodiment of anoximeter, the oximeter may be designed to operate with a selectedwavelength within that range, for example, 670 nm. However, the LEDsmanufactured to produce the selected wavelength of 670 nm involvemanufacturing tolerances typically in the range of .+−0.2-10 nm for thesame drive current. However, for a typical LED used in oximetry, thedrive current can be varied in order to obtain the desired outputwavelength for the LED. For instance, as illustrated in FIG. 3B, therepresented LED has an operating wavelength of 660 nm for the typical 50mA drive current. If the drive current is increased to approximately 85mA, the operating wavelength becomes the selected wavelength of thepresent example (670 nm). The present invention takes advantage of theobserved wavelength shift in response to a drive current change to tuneeach LED to obtain the selected wavelength, such as 670 nm.

For purposes of discussion, the first LED 160 is defined to exhibit thewavelength characteristic depicted in FIG. 3B. To tune the first LED160, the drive current from the LED driver 182 is assumed to be presetor fixed. In the present embodiment, the drive current is preferablysomewhat larger than the drive current necessary to drive the first LED160 alone (e.g., 100 mA or more). This is because the first tuningresistor 162 carries some of the fixed drive current from the LED driver182. The first tuning resistor 162 is selected to draw an appropriateamount of the fixed drive current to adjust the amount of currentflowing through the first LED 160 to result in the selected outputwavelength. In the present example, the resistor is chosen to carryapproximately 15 mA (of the 100 mA from the LED driver 182) in order toreduce the current through the first LED 160 to approximately 85 mA toobtain the 670 nm selected wavelength. Accordingly, each LED can bedriven with the same fixed drive current from the LED driver 182, yetthe current through any particular LED differs in accordance with thevalue of the associated tuning resistor. In this manner, the LED driver182 can be designed to provide the same fixed drive current for everysensor connected to the oximeter. The oximeter system 152 is thusdesigned to make its calculation based on the assumption that thecorresponding wavelengths remain constant from sensor to sensor.

One particular advantageous method of selecting the tuning resistorinvolves the use of a semiconductor substrate resistor, such as theresistor 210 depicted in FIGS. 5A and 5B. The resistor 210 depicted inFIG. 5A comprises a semiconductor substrate 212, a resistive coating pad214, and connective conductors 216, 218. In one embodiment a tunable LED220 (i.e., an LED that exhibits wavelength shift with drive currentchange) is connected in parallel with the semiconductor substrateresistor 210. The fixed (preset) drive current is then applied with acurrent source 222 to the network formed by the substrate resistor 210and the tunable LED 220. The operating wavelength of the tunable LED 220is measured. Preferably, the initial substrate resistor has lessresistance than will be necessary to obtain the desired outputwavelength. A laser is used to scribe the resistive pad 214, as depictedby the line 224 in FIG. 5B. The scribe line 224 effectively removes aportion of the resistive pad 214, and thereby increases the resistanceof the remaining resistive pad 214, as well known in the art. Using thelaser, the increase in resistance can be controlled very precisely. Theresistive pad 214 can be laser trimmed until the current through thetunable LED 220 causes the tunable LED 220 to generate the selectedoperating wavelength. The resulting resistor/LED pair forms a tuned LEDnetwork. This tuning method is advantageous because of the precision andthe resulting low-cost of the tuned LED.

Other methods of selecting the first tuning resistor 162, such ascalculating the wavelength shift for a given current change for thefirst LED 160, and then selecting the appropriate resistor to cause thecorrect amount of current to flow through the LED to obtain the selectedoperating wavelength, can also be used. Similarly, a potentiometer couldbe used. Preferably, each LED for each sensor is tuned in a similarmanner such that the operating wavelength is a selected operatingwavelength for the sensor. For instance, a two wavelength oximeteroperating may have selected wavelengths for the two LEDs of 670 nm and905 nm. For each sensor, a first LED is tuned for the 670 nm selectedwavelength, and a second LED is tuned for the 905 nm selectedwavelength.

In sum, the tuning aspect of the present invention involves using theprinciple of wavelength shift in an LED to tune each LED to obtain arespective selected operating wavelength.

It should be understood that for some LEDs, the manufacturing tolerancemay be too far from the respective selected wavelength to enable the useof the shift in wavelength to properly tune the LED; or the wavelengthshift may be insufficient to obtain the selected wavelength. In oneembodiment, such LEDs would not be utilized, and would be considered outof tolerance. Alternatively, if the obtainable wavelength shift is notsufficient to allow for proper tuning, it is also possible to use twoLEDs having wavelengths very near each other and near the selectedwavelength. One LED has a wavelength below the selected wavelength, andone LED has a wavelength above the selected wavelength. As the graph ofFIG. 6 illustrates, when two LEDs are both active and placed adjacentone another, the light from the two LEDs combines to form a combinedwavelength which is the average wavelength of the two LEDs. The combinedwavelength has a broader wavelength range, but has a known average.Preferably, to fine tune the average wavelength, the wavelength shift ofone or both of the two LEDs can be utilized using tuning resistors asdescribed above such that the average wavelength is the selectedwavelength. Accordingly, two LEDs (preferably tuned in accordance withthe present invention as a pair) can be used to obtain the selectedwavelength for operation in a given oximeter.

As another alternative, if sufficient wavelength shift is not availableto allow for tuning all LEDs to the selected wavelengths, a few selectedwavelengths could be used. For instance, for determining oxygensaturation, the selected red wavelengths could be 660 nm, 670 nm and 680nm. The selected infrared wavelengths could be 900 nm, 920 nm, and 940nm, independent of the red wavelengths. Each sensor would be tuned usingthe tuning resistors described above such that the red and infrared LEDsoperate at one of the selected red and infrared wavelengths,respectively. An indicator would then be provided on the sensor, or theconnector attached to the sensor, to allow the oximeter to determinewhich of the selected wavelengths is present on the sensor attached tothe oximeter. Alternatively, a wavelength detection device could beprovided with the oximeter system to determine which of the selectedwavelengths is present in a sensor attached to the oximeter system.Although this embodiment requires some means for the oximeter todetermine which of the selected wavelengths is present on the attachedsensor, the selected wavelengths are precise from sensor to sensor.

Two-Wavelength LED

Another aspect of the present invention involves using the principle ofwavelength shift in an LED for a given change in current in order to usea single LED to provide two operating wavelengths. This is advantageousin making physiological measurements, such as blood oximetrymeasurements, because for each additional wavelength added, thesaturation of an additional constituent in the blood can be measured.For instance, with a two-wavelength oximeter, only the ratio of one oftwo constituents to the total of the two constituents (e.g., oxygensaturation) can be accurately monitored. If oxygen saturation ismonitored with two wavelengths, other constituents which aresignificantly present in the blood affect the measurement of oxygensaturation.

If an additional constituent present in the blood has a significanteffect upon the oxygen saturation reading for a particular patient, thefailure to detect the constituent can be detrimental to the patient. Anexample of a constituent which, when present in the blood, willsignificantly impact the oxygen saturation reading provided by atwo-wavelength oximeter is carbon monoxide. This is because theextinction coefficient magnitude for carboxyhemoglobin (depicted in thecurve 106 of FIG. 2) approaches the extinction coefficient ofoxyhemoglobin (depicted in the curve 102 of FIG. 2) for light energy inthe range of 660 nm. Therefore, carboxyhemoglobin may be detected asoxyhemoglobin. This leads to a false indication of the oxygen saturation(i.e., overestimation) in the blood using a two-wavelength oximeter. Inthis manner, the attending physician may fail to detect the lack ofoxygen, and the increase of carbon monoxide in a patient. If anadditional transmission wavelength is provided on the sensor, theoximeter can monitor another constituent, such as carboxyhemoglobin.

In accordance with the present invention, the principle of wavelengthshift in an LED is utilized in order to drive one LED with twoappropriate drive current levels to provide two distinct wavelengths. Inits simplest form, this is accomplished by first driving an LED (whichexhibits wavelength shift with drive current change) with a first knowndrive current to a first known wavelength, and then driving the same LEDwith a second known current to a second known wavelength.

FIG. 7 depicts one advantageous embodiment of a sensor 250 for bloodoximetry measurements coupled to an oximeter system 252 designed inaccordance with this aspect of the present invention. The sensor 250comprises a first LED 254 and a second LED 256. For blood oximetry thefirst LED 254 preferably operates in the red wavelength range and thesecond LED 256 preferably operates in the infrared wavelength range. Thesensor 250 further comprises a photodetector 258. The photodetector 258is coupled to receiving and conditioning circuitry 262. The oximetersystem is under the control of a controller 264 and has a display 266.As well-understood in the art, an LED driver 260 sequentially drives theLEDs 254, 256 with a predetermined drive current. The photodetector 258detects the light energy, attenuated by the medium under test. Theoximeter 252 receives arid analyzes the signal from the photodetector258 to determine information regarding the medium through which thelight energy has been transmitted. As with the embodiment of FIG. 4, theoximeter system 252 is depicted in simplified form. Appropriate oximetersystems include the system disclosed in copending U.S. patentapplication Ser. No. 08/320,154, filed Oct. 7, 1994, which has beenassigned to the assignee of the present application. Other monitors wellunderstood in the art also exist. The oximeter system 252 is modified inaccordance with the present invention to drive the shifting LED asdescribed below.

In the present example for blood oximetry, the first LED 254 is theshifting LED and is used to provide two wavelengths. In order toaccurately provide two wavelengths, the wavelength shift principle isutilized. According to one embodiment, LEDs are evaluated at the time asensor is manufactured, and an indicator is provided on the sensor whichcan be read by the oximeter system 252 to indicate the drive currentchange necessary in order to effectuate a desired shift in wavelength.Indicators may comprise a resistor on the sensor or sensor connector, amemory on the sensor or sensor connector, or a similar device.Alternatively, the indicator can provide a indication to the oximeter ofthe amount of wavelength shift which is obtained due to a preset drivecurrent change. Another alternative is to provide a wavelength detector268 for the oximeter, which allows the oximeter system 252 to detect thetransmission wavelength of an active LED. Wavelength detectors, such asa monochrometer, are well known in the art. However, conventionalmonochrometers are expensive and bulky. This description sets forth amore practical approach to detecting wavelength below. In thisembodiment, the LED driver 260 changes the drive current until thedesired wavelength is obtained, utilizing the wavelength detector 268 tomonitor the wavelength.

In one preferred embodiment allowing for a simpler oximeter design, inorder to accurately provide two wavelengths with a single LED such asthe first LED 254, a network 270 of a slope adjusting resistor 272 andthe first LED 254 is slope adjusted such that a preselected change indrive current (ΔI) entering the first slope adjusted network, causes apreselected shift in wavelength (Δλ) in the first LED 254. In otherwords, as depicted in FIG. 3B, each LED exhibits an inherent slope ofthe curve 122. However, the slope of this curve often differs from LEDto LED, even for LEDs rated for a particular wavelength. In order for anoximeter to be designed for simplicity in obtaining a repeatablepreselected wavelength shift, it is advantageous to have the preselectedwavelength shift (Δλ) for each first LED in different sensors correspondto the same preselected drive current change (ΔI). Accordingly, it isdesirous that the first LED (for the present example) on differentprobes respond with the same preselected change in wavelength for thesame change in drive current provided by the LED driver 260. In otherwords, it is advantageous that the slope of the curve 100 depicted inFIG. 3B be the same for each corresponding LED network, since it is nottypically the same for each individual LED. In this manner, the oximeteris designed to drive the LEDs with two drive, current levels, where thetwo drive current levels are preselected and remain constant from sensorto sensor.

Just as the first tuning resistor 162 tunes the first LED 160 to aparticular selected wavelength for a selected drive current, a slopeadjusting resistor, such as the slope adjusting resistor 272, can beused to alter the slope of the curve 122 exhibited for the particularcorresponding LED network (e.g., the first slope adjusted LED network270). In most instances, the slope adjusting resistor 272, if used toalter the slope, cannot also be used to tune the precise wavelength ofthe first LED 254. However, other methods and procedures to indicate tothe oximeter what the particular wavelength of operation of the firstLED for a given drive current can be utilized. For instance, anindicator (such as a resistor or low cost memory device) can be providedwith the sensor 250 which can be read by the oximeter 252, whichindicator provides the initial operating wavelength of the slopeadjusted LED network 270.

Slope adjustment can be accomplished in the same manner as describedabove with respect to the semiconductor substrate resistor 210. However,the substrate resistor functions as the slope adjusting resistor ratherthan a wavelength tuning resistor (i.e., the substrate resistor isadjusted to cause a preselected change in wavelength for a preselectedchange in drive current for the LED/resistor network). In other words,for the first LED 254, the substrate resistor 210 depicted in FIGS. 5Aand 5B is coupled to the first LED 254 to form the slope adjustingresistor 272. A laser is used to trim the resistor until the preselectedchange in drive current for the network 270 results in the preselectedchange in wavelength for the first LED 254.

It should be noted that if LEDs are available that exhibit the samewavelength shift with respect to the same change in drive current, thefirst slope adjusting resistor 272 is unnecessary.

For determining oxygen saturation, the second LED 256 operates at afixed infrared wavelength (e.g., 905 nm). Preferably, if the infraredLEDs exhibit manufacturing tolerances, the infrared LEDs can be tunedusing a tuning resistor 274, in the same manner as the tuning resistor162 of FIG. 4, to operate at the selected infrared wavelength. With atuned second (infrared) LED 256 and a slope adjusted first LED 254(configured to provide two wavelengths), measurements at threewavelengths can be taken using the sensor 250.

In use, the sensor 250 of FIG. 7 is first driven with an initial drivecurrent to cause the first LED 254 to generate light energy of a firstwavelength (e.g., 660 nm). The attenuated signal at this firstwavelength is detected by the photodetector 258 and received by theoximeter 252. Next, the first slope adjusted LED 254 is driven with anew drive current varied by the preselected change in drive current tocause the preselected wavelength shift to obtain a second wavelength(e.g., 675). As long as the initial wavelength is provided to theoximeter system 252, and the slope (change in wavelength due to changein current) of the first LED network 270 is properly adjusted to matchthe preselected slope, the second wavelength will also be a knownquantity. A third measurement is taken by driving the second LED 256 andreceiving the attenuated signal with the photodetector 258. Measurementsare stored in the oximeter system 252. Based upon the three measurementstaken, the arterial saturation of two constituents of blood may bedetermined (e.g., oxyhemoglobin and carboxyhemoglobin), thus providingmore precise information regarding the physiological makeup of the bloodof a patient under test.

In an oximeter system where monitoring of carbon monoxide and oxygen isdesired, the first wavelength may be 660 nm, the second wavelength maybe 675 nm or 680 nm and the third wavelength will be an infraredwavelength such as 900 nm or 905 nm. With these three wavelengthsprovided by two LEDs, the saturation of both oxyhemoglobin andcarboxyhemoglobin in blood can be determined. The use of two LEDs toperform measurements at three wavelengths reduces the cost of thesensor, which is particularly advantageous if the sensor is a disposableor replaceable sensor.

In addition to the uses described above, it should also be noted thatthe wavelength shift principal described above could be used to obtain,an additional wavelength with one LED for use in the ratiometric methodof determining blood oxygen as described in copending U.S. patentapplication Ser. No. 07/672,890, filed Nov. 21, 1991, which has beenassigned to the assignee of the present application.

Measurements without Precise Wavelength Information

A further aspect of the present invention involves an apparatus andmethod of measuring the saturation of a selected constituent in a mediumunder test (e.g., oxyhemoglobin in blood) without knowing the preciseoperational wavelength of one LED. According to this aspect of thepresent invention, if the wavelength shift for an LED is known for aknown change in drive current, the operational wavelength for the LEDneed not be known if other information is also available, as furtherexplained below.

As explained above, obtaining a known wavelength shift for a selectedchange in current can be accomplished by adjusting presently existingLEDs, such that the LEDs react to a preselected change in drive current(ΔI) with a preselected change in wavelength (Δλ). Alternatively, ifLEDs are available having a repeatable (from LED to LED) change inwavelength for a selected change in current, those LEDs can be usedwithout adjustment. An understanding of this aspect of the presentinvention is explained with reference to arterial oxygen saturationdetermination using two-wavelength oximeters.

As explained above, FIG. 2 depicts a graph illustrating the relationshipbetween the typical extinction coefficient for three constituents ofblood with respect to the transmission wavelength of light transmittedthrough the blood. For purposes of determining oxygen saturation, thefirst curve 102 and second curve 104 are of interest.

As illustrated by the first curve 102, the extinction coefficient ofoxyhemoglobin for light transmitted between approximately 665 nm(indicated as λ₁ on the graph) and 690 nm (indicated as λ₂ on the graph)is substantially constant (more apparent when the Y-axis of FIG. 2 isnot a log scale axis). When light within that same range (i.e., λ₁-λ₂)is transmitted through reduced hemoglobin (the second curve 104), theextinction coefficient of the reduced hemoglobin exhibits asubstantially linear relationship as a function of transmissionwavelength. These known properties of blood constituents are utilized inthe apparatus and method of the present invention to obtain informationregarding the oxygen saturation (or other constituent saturation) of theblood without knowing the particular wavelength of one of two LEDs.

Assuming that incident light is represented by the letter 10 and theattenuated signal is represented by I, the attenuated signal isrepresented by Equation (1) above. In other words, for the LED sensor250 of FIG. 7, the attenuated signal I is received by the photodetector258 and is a function of the ambient transmission, as set forth inEquation (1).

Where light of wavelength λ is transmitted through tissue with bloodcontaining two forms of hemoglobin (oxyhemoglobin and reducedhemoglobin), Equation (1) can be expanded for these two constituents ofblood:

$\begin{matrix}{I = {{I_{0}\left( ^{- {\sum\limits_{j = 1}^{a}{ɛ_{j}d_{j}c_{j}}}} \right)}\left( ^{{- d}\; ɛ_{1\; \lambda}c_{1}} \right)\left( ^{{- d}\; ɛ_{2\; \lambda}c_{2}} \right)}} & (2)\end{matrix}$

where:

d is the thickness of the medium.

ε_(1λ) is the absorption coefficient of reduced hemoglobin at wavelengthλ,

ε_(2λ) is the absorption coefficient of oxyhemoglobin at wavelength λ,

c₁ is the concentration of reduced hemoglobin,

c₂ is the concentration of oxyhemoglobin,

ε_(j) is the absorption coefficient of the j^(th) layer of attenuatingmaterial (not including oxyhemoglobin and reduced hemoglobin),

d_(j) is the thickness of the j^(th) layer of attenuation material (notincluding oxyhemoglobin and reduced hemoglobin), and

c_(j) is the concentration of the j^(th) layer of attenuating material(not including oxyhemoglobin and reduced hemoglobin).

Equation (2) can be further expressed as follows:

$\begin{matrix}{{s = {{\ln \left( \frac{I}{I_{BL}} \right)} = {- {d\left( {{ɛ_{1\; \lambda}c_{1}} + {ɛ_{2\; \lambda}c_{2}}} \right)}}}}{{where}\text{:}}{I_{BL} = {{I_{0}\left( ^{- {\sum\limits_{j = 1}^{a}{ɛ_{j}d_{j}c_{j}}}} \right)} = {baseline}}}} & (3)\end{matrix}$

s is a value obtained by measuring I with the photodetector andcalculating the ratio of I to I_(BL) after taking the natural log.

For determining oxygen saturation, where the light is transmitted at afirst red wavelength λ₁, Equation (3) is expressed as follows:

$\begin{matrix}{s_{1} = {{{\ln \left( \frac{I}{I_{BL}} \right)}_{\lambda_{1}}} = {- {d\left( {{ɛ_{1\; \lambda_{1}}c_{1}} + {ɛ_{2\; \lambda_{1}}c_{2}}} \right)}}}} & (4)\end{matrix}$

Where light is transmitted at an infrared wavelength λ_(IR), Equation(3) is expressed as follows:

$\begin{matrix}{s_{IR} = {{{\ln \left( \frac{I}{I_{BL}} \right)}_{\lambda_{1}}} = {- {d\left( {{ɛ_{1\; \lambda_{1}}c_{1}} + {ɛ_{2\; \lambda_{1}}c_{2}}} \right)}}}} & (5)\end{matrix}$

When the wavelength λ₁ and the wavelength λ_(IR) are both known, theoxygen saturation can be determined, as well-understood in the art. Thisis briefly illustrated with the following derivation:

$\begin{matrix}{{{LET}\mspace{14mu} N_{1}} = {{\frac{S_{1}}{d}\mspace{20mu} {and}\mspace{14mu} N_{2}} = \frac{S_{IR}}{d}}} & (6)\end{matrix}$

Equations (4) and (5) become:

N ₁ =c ₂ε_(2λ) ₁ +c ₁ε_(1λ) ₁   (7)

N ₂ =c ₂ε_(2λ) _(IR) +c ₁ε_(1λ) _(IR)   (8)

In matrix notation, Equations (7) and (8) become:

$\begin{matrix}{{A = \begin{pmatrix}ɛ_{2\lambda_{1}} & ɛ_{1\; \lambda_{1}} \\ɛ_{2\lambda_{IR}} & ɛ_{2\lambda_{IR}}\end{pmatrix}}{X = \begin{pmatrix}C_{2} \\C_{1}\end{pmatrix}}{B = \begin{pmatrix}N_{1} \\N_{2}\end{pmatrix}}{{A \cdot X} = {\left. B\Rightarrow{\begin{pmatrix}ɛ_{2\lambda_{1}} & ɛ_{1\lambda_{1}} \\ɛ_{2\lambda_{IR}} & ɛ_{2\lambda_{IR}}\end{pmatrix}\begin{pmatrix}C_{2} \\C_{1}\end{pmatrix}} \right. = \begin{pmatrix}N_{1} \\N_{2}\end{pmatrix}}}{{Or}\text{:}}{\begin{pmatrix}C_{2} \\C_{1}\end{pmatrix} = {\begin{pmatrix}ɛ_{2\lambda_{1}} & ɛ_{1\lambda_{1}} \\ɛ_{2\lambda_{IR}} & ɛ_{2\lambda_{IR}}\end{pmatrix}^{- 1}\begin{pmatrix}N_{1} \\N_{2}\end{pmatrix}}}} & (9)\end{matrix}$

Hence:

$\begin{matrix}{\begin{pmatrix}C_{2} \\C_{1}\end{pmatrix} = \begin{bmatrix}\frac{\left( {{ɛ_{1\lambda_{IR}}N_{1}} - {ɛ_{1\lambda_{1}}N_{2}}} \right)}{\left( {{ɛ_{2\lambda_{1}}ɛ_{1\lambda_{IR}}} - {ɛ_{1\lambda_{1}}ɛ_{2\lambda_{IR}}}} \right)} \\\frac{\left( {{{- ɛ_{2\lambda_{1}}}N_{1}} + {ɛ_{2\lambda_{1}}N_{2}}} \right)}{\left( {{ɛ_{2\lambda_{1}}ɛ_{1\lambda_{IR}}} - {ɛ_{1\lambda_{1}}ɛ_{2\lambda_{IR}}}} \right)}\end{bmatrix}} & (10)\end{matrix}$

As well understood in the art, oxygen saturation is defined as thefollowing ratio:

oxygen:

$\begin{matrix}\begin{matrix}{{SAT} = \left. \frac{C_{2}}{C_{2} + C_{1}}\Rightarrow\frac{1}{SAT} \right.} \\{= \frac{C_{2} + C_{1}}{C_{2}}}\end{matrix} & (11)\end{matrix}$

Or:

${SAT} = {1 + \frac{C_{1}}{C_{2}}}$

Hence:

$\frac{C_{1}}{C_{2}} = \frac{\frac{\left( {{{- ɛ_{2\lambda_{1}}}N_{1}} + {ɛ_{2\lambda_{1}}N_{2}}} \right)}{\left( {{ɛ_{2\lambda_{1}}ɛ_{1\lambda_{IR}}} - {ɛ_{1\lambda_{1}}ɛ_{2\lambda_{IR}}}} \right)}}{\frac{\left( {{ɛ_{1\lambda_{IR}}N_{1}} - {ɛ_{1\lambda_{1}}N_{2}}} \right)}{\left( {{ɛ_{2\lambda_{1}}ɛ_{1\lambda_{IR}}} - {ɛ_{1\lambda_{1}}ɛ_{2\lambda_{IR}}}} \right)}}$

Substituting:

$N_{1} = {{\frac{S_{1}}{d}\mspace{14mu} {and}\mspace{14mu} N_{2}} = \frac{S_{IR}}{d}}$

and multiplying the numerator and denominator by −1:

and simplifying:

$\frac{C_{1}}{C_{2}} = {ɛ_{2\lambda_{1}}\frac{\left( {\frac{S_{1}}{d} - \frac{S_{IR}}{d}} \right)}{\left( {{{- ɛ_{1\lambda_{IR}}}\frac{S_{1}}{d}} + {ɛ_{1\lambda_{1}}\frac{S_{IR}}{d}}} \right)}}$

Multiplying numerator and denominator by d:

$\begin{matrix}{\frac{C_{1}}{C_{2}} = {ɛ_{2\lambda_{1}}\frac{\left( {S_{1} - S_{IR}} \right)}{\left( {{{- ɛ_{1\lambda_{IR}}}S_{1}} + {ɛ_{1\lambda_{1}}S_{IR}}} \right)}}} & (12)\end{matrix}$

Substituting Equation (12) into Equation (11) above:

$\frac{1}{SAT} = {{ɛ_{2\lambda_{1}}\frac{\left( {S_{1} - S_{IR}} \right)}{\left( {{{- ɛ_{1\lambda_{IR}}}S_{1}} + {ɛ_{1\lambda_{1}}S_{IR}}} \right)}} + 1}$

Simplifying:

$\frac{1}{SAT} = \frac{\left( {{ɛ_{2\lambda_{1}}S_{1}} - {ɛ_{2\lambda_{1}}S_{IR}} - {ɛ_{1\lambda_{IR}}S_{1}} + {ɛ_{1\lambda_{1}}S_{IR}}} \right)}{\left( {{{- ɛ_{1\lambda_{IR}}}S_{1}} + {ɛ_{1\lambda_{1}}S_{IR}}} \right)}$

And finally:

$\begin{matrix}{{SAT} = \frac{\left( {{ɛ_{1\lambda_{IR}}S_{1}} + {ɛ_{1\lambda_{1}}S_{2}}} \right)}{\left( {{{- ɛ_{2\lambda_{1}}}S_{1}} + {ɛ_{2\lambda_{1}}S_{2}} + {ɛ_{1\lambda_{IR}}S_{1}} - {ɛ_{1\lambda_{1}}S_{2}}} \right)}} & (13)\end{matrix}$

When the wavelength λ₁ and the λ_(IR) are both known, the extinctioncoefficients, ε_(1λ) ₁ , ε_(2λ) ₁ , ε_(1λ) _(IR) and ε_(2λ) _(IR) , forthe corresponding constituents at λ₁ and λ_(IR) are also known. Asexplained above, S₁ and S_(IR) can be obtained by measuring I and I₀ andtaking the natural log of this ratio at the various wavelengths duringoperation. Accordingly, all of the variables in the saturation equationare known or obtainable through measurement.

However, if the wavelengths for the transmission LEDs are notspecifically known, the extinction coefficients E will not be known. Inaccordance with one aspect of the present invention, the oxygensaturation can be computed without knowing the precise wavelength of oneof the LEDs. For purposes of discussion herein, the LED in the red rangeis chosen for illustration of this aspect of the present invention. Inaccordance with the present invention, and as explained above, the redLED can be adjusted to exhibit a preselected wavelength shift, eventhough the precise wavelength may not be known. Accordingly, the red LEDcan be driven with two different drive currents to obtain two differentwavelengths, the shift between which is preselected and known. However,as explained above, the precise wavelength may be unknown without someindication of at least the starting wavelength. In accordance with thepresent invention, as long as the preselected wavelength shift is known,the starting wavelength need not be known.

In an application where the extinction coefficients vary with respect toshifts in wavelength on the order of 1-3 nm, it would be possible todetermine the wavelength without prior information regarding thewavelength or the wavelength shift. This would be accomplished bycalculating the desired measurement (e.g., oxygen saturation) at several(e.g., two or more) different LED drive currents and using the change inthe measurement in connection with an empirically generated data set(i.e., curves) of measurements with respect to wavelengths to determinethe wavelength of the LED.

If the preselected wavelength shift is utilized, the oximeter system canmake measurements at three wavelengths λ₁, λ₂ and λ_(IR). Thus, a thirdequation in addition to Equations (3) and (4) is obtained.

Where the light is transmitted at a second red wavelength λ₂, Equation(3) is expressed as follows:

$\begin{matrix}\begin{matrix}{{s_{2} = {\ln \left( \frac{I}{I_{BL}} \right)}}}_{\lambda_{2}} \\{= {- {d\left( {ɛ_{1\lambda_{2}c_{1}} + {ɛ_{2\lambda_{2}}c_{2}}} \right)}}}\end{matrix} & (14)\end{matrix}$

As depicted in FIG. 2, within the range of 650 nm-700 nm, the extinctioncoefficient does not significantly change. More particularly, within therange of λ₁−λ₂=665 mm-690 mm,

ε_(2λ) ₂ ≅ε_(2λ) ₁   (15)

Furthermore within the same range,

ε_(1λ) ₂ =(ε_(1λ) ₁ −Δε₁)  (16)

Δε₁ is known for a known wavelength shift within the described range,because the change in the extinction coefficient Δε₁ is substantiallylinear.

Substituting Equations (14) and (15) into Equation (4), (5), and (14)results in the following equations:

S ₁ =−d(ε_(1λ) ₁ c ₁+ε_(2λ) ₂ c ₂)  (17)

S _(IR) =−d(ε_(1λ) _(IR) c ₁+ε_(2λ) _(IR) c ₂)  (18)

S ₂ =−d(ε_(1λ) ₁ −Δε₁)c ₁+ε_(2λ) ₂ c ₂)  (19)

As explained above, S₁, S₂, and S_(IR) are calculated by measuring I andI_(BL). Accordingly, S₁, S₂, and S_(IR), are known values. Theextinction coefficients ε₁ and ε₂ for the infrared wavelength LED areassumed to be known because in the infrared wavelength of interest(e.g., 850 nm-920 nm) and more particularly 890 nm-910 nm), theextinction coefficient is substantially constant for both curves 102 and104. In another embodiment, the accuracy would be improved slightly bytuning the LED. The extinction coefficients for oxyhemoglobin at λ₁ andλ₂ are also known, as long as the wavelength is in the range where theextinction coefficient remains constant. In the present example, thisrange is defined as 665 nm to 690 nm. Furthermore, because the change inthe absorption coefficient (Δε₁) for reduced hemoglobin is unknown for aknown wavelength shift between λ₁−λ₂=665 nm-690 nm, Δε₁ is also a knownquantity because ε₁ is linear with λ. The total thickness of the medium,d, generally is unknown for most applications. However, for thedetermination of oxygen saturation, as illustrated above, the thickness(d) cancels because saturation is a ratio.

Accordingly, for the determination of oxygen saturation, Equations (17),(18), and (19) provide three equations with three unknowns (ε_(1λ) ₁ ,c₁ and c₂). Algebraic techniques following those of Equations (6) to(13) may be applied to solve the three equations to obtain the oxygensaturation ratio of c₂/(c₁+c₂). Accordingly, it is not necessary to knowthe precise operating wavelength of the first LED 254, as long as theoperating wavelength for the first LED 254 is in a known range where apreselected change in drive current causes a preselected change in thewavelength, and where the extinction coefficient of one constituent isconstant and the extinction coefficient of the second constituent issubstantially linear such that the change in the extinction coefficientfor a preselected change in wavelength is also known.

Accordingly, this aspect of the present invention permits the user toobtain physiological data without knowing the precise operationalfrequency of an LED.

Improved Calibration of LED Sensor

An additional aspect of the present invention involves an improvedcalibration technique for an oximeter sensor where a resistor isutilized to code the LED rather than tune the LED. As depicted in theprior art calibrated oximeter probe of FIG. 1, an encoding resistor 300utilizes a separate electrical connection lead and connects to a commonground lead 304. With the ever increasing use of replaceable ordisposable sensors, any reduction in the complexity of the replaceablesensor can result in a significant cost savings over time. In accordancewith present invention, the characteristics of an LED as depicted inFIG. 3A can be utilized to provide a more cost effective coded orcalibrated oximeter probe where the coding or calibration is providedusing a coding resistor.

In accordance with this aspect of the present invention, one of the LEDelectrical connections can also be used for the coding resistor. FIG. 8depicts a schematic diagram of an exemplary oximeter sensor where acoding resistor 332 can be read using one of the LED electricalconnections rather than a separate electrical connection. A sensor 310comprises a first LED 312, a second LED 314 and a photodetector 316. Thefirst LED 312 has a first corresponding electrical connection 318; thesecond LED 314 has a second corresponding electrical connection 320; andthe photodetector 316 has a corresponding electrical connection 322.Each of the LEDs 312, 314 and the photodetector 316 are connected attheir outputs to a common ground electrical connection 330. In thepresent embodiment, the coding resistor 332 is coupled in parallel withthe first LED 312 or the second LED 314. In this embodiment, the codingresistor 332 is not provided to tune the first LED 312 or to slopeadjust the first LED network, but is provided as an indicator which canbe read by an attached oximeter system 340. The resistor can be used toindicate the operating wavelength of the first and second LEDs 312, 314,or more advantageously, to indicate the type of probe. In other words,the value of the coding resistor 332 can be selected to indicate thatthe probe is an adult probe, a pediatric probe, a neonatal probe, adisposable probe or a reusable probe. In one preferred embodiment,coding resistors could be provided across each of the LEDs 312, 314 toallow additional information about the probe to be coded without addedleads. However, any resistor or impedance device could be used withoutit being used in parallel with the LEDs to encode the change inwavelength or other information for the LEDs.

For instance, the coding resistor could be utilized for securitypurposes. In other words, the value of the coding resistor, and theplacement across the LED 312 could be used to ensure that the probe isconfigured properly for the oximeter. For instance, the coding resistorcould be utilized to indicate that the probe is from an authorizedsupplier such as a “Masimo” standard probe, “Patient Monitoring Company1” probe, “Patient Monitoring Company 2” probe, etc.

In addition, it should be noted that the resistor need not be a passiveelement. Coding information could also be provided through an activecircuit such as a transistor network, memory chip, or otheridentification device, for instance Dallas Semiconductor DS 1990 or DS2401 or other automatic identification chip.

In order to read the coding resistor 332, the oximeter system 340 drivesthe first LED 312/coding resistor 332 combination at a level that is lowenough that the LED draws effectively insignificant current because ofthe exponential relationship between I and V, as illustrated in thegraph of FIG. 3A. As well understood in the art, the LED becomes activein the area of the shoulder, designated with the A axis indicator. Belowthe voltage level at A, the LED is effectively inactive and drawseffectively insignificant current. In other words, the current throughthe first LED 312 is negligible. Significantly all of the currentthrough the first electrical connection 318 flows through the codingresistor 332.

The current which flows through the coding resistor for the voltageapplied is measured by the oximeter system by measuring the currentthrough the first electrical connection 318. In turn, the oximetersystem 340 determines the value of the coding resistor 332 which ispreselected to indicate the type of probe, the operating wavelength orother parameters about the probe. In essence, by reducing the drivevoltage across the first electrical connection 318 and ground to a lowlevel that does not activate the first LED 312, the first LED 312 iseffectively removed from the electrical circuit. In the presentembodiment, it has been found that for conventional LEDs in the red andIR range, 0.5V is a particularly advantageous voltage. At 0.5V, currentthrough the LED is generally less than 1 μA (an insignificant amount).

Preferably, the coding resistor 332 is chosen to be of a sufficientlyhigh value that when the current supply to the first electricalconnection 318 rises to a level sufficient to drive the first LED 312,the coding resistor 332 is effectively removed from the electricalcircuit because of its high resistance as compared to the resistance ofthe first LED 312 at active operating currents.

Accordingly, a coding resistor can be used in connection with anoximeter LED sensor without the addition of an electrical connectordedicated to the coding resistor. This reduces the cost of the sensor inaccordance with the present invention.

In one advantageous embodiment, the oximeter can monitor the codingresistor continuously by providing a 0.5V coding resistor reading signalat a frequency different from the LED drive current. For instance, ifthe LED drive current is turned on and off at a frequency of 625 Hz, the0.5V coding resistor reading voltage can be provided at a frequency muchlower than 625 Hz, such that the 625 Hz signal can be easily filteredwith a low pass filter with a cutoff significantly below 625 Hz, butwith a pass band which allows the 0.5V signal to pass. This would allowthe oximeter to continuously monitor the coding resistor 332 in case ofa change in the sensor by the system operator.

This particularly advantageous embodiment of using the coding resistor332 can also be utilized with a conventional back-to-back configurationfor the red and infrared LEDs, as is typical in oximeters. Such aconfiguration is depicted in FIG. 8A. FIG. 8A is similar to FIG. 8,except that the first LED 312 and the second LED 314 are connected in aback-to-back configuration such that the first electrical connection 318is required and the voltage can be alternated from positive to negativeto draw current through either the second LED 314 or the first LED 312.This eliminates the need for an electrical connection to the oximeterprobe, thereby further reducing the cost of the probe. In theback-to-back configuration of FIG. 8A, if the second LED 314 is a redLED with a knee of approximately 2.0V and that the second LED 312 is aninfrared (IR) LED with a knee of approximately 1.5V, a positive voltageis advantageously applied to the first electrical connection 318 atapproximately 0.5V in order to measure the coding resistor 332. Becausethe knee for the red LED is 2.0V, very little (less than 1 μA) currentwill flow through the red LED and essentially no current will flowthrough the infrared LED 312 (because the infrared LED 312 is reversebiased). In such a scenario, the current which passes through thenetwork of the first LED 312, the second LED 314, and the codingresistor 332 is approximately equal to the current through the codingresistor 332. The resistance of the coding resistor 332 is then easilydetermined via Ohms Law by dividing the voltage applied to the networkby the current which flows through the network. Care must be taken toinsure that the element (active or passive) does not createelectromagnetic noise which could lead to reduced system signal to noiseratio.

Wavelength Detection

As briefly discussed above, in certain circumstances, it is usefuldirectly to obtain information regarding the wavelength of an LEDconnected to an oximeter. As illustrated in FIG. 7, a wavelengthdetector 268 can be provided. However, a wavelength detector requiressome configuration operations to be performed by the operator. In ahospital environment, it is advantageous to simplify the use of theoximeter. Accordingly, in another embodiment, each LED sensor isconfigured with a wavelength detection configuration. FIGS. 9A and 9Bdepict diagrams of possible embodiments of LED sensors configured withfilters. These sensor configurations can be used to obtain thewavelength of the LED for the sensor.

As depicted in FIG. 9A, a sensor 400 comprises a transmission LEDnetwork 402, a first photodetector 404, a second photodetector 406, adiffuser 407, a beam splitter 408, an optical filter 410 and an optionaloptical filter 471. The transmission LED network 402, the firstphotodetector 404 and the second photodetector 406 all couple to anoximeter system 412. A third photodetector 413 is also depicted indotted line to illustrate the photodetector for the oximetrymeasurement. This third photodetector 413 is not discussed in thefollowing discussion which relates to the calibration portion of theoximeter probe 400. The transmission LED network 402 preferablycomprises at least two LEDs, one in the red wavelength range (e.g., 660nm) and one in the infrared wavelength range (e.g., 905 nm). Determiningthe wavelength of one of the LEDs in the LED network 402 using theconfiguration of the sensor 400 depicted in FIG. 9A is described below.

As seen in FIG. 9A, the LED network 402 transmits light 414 which firstpasses through the diffuser 407. The diffuser 407 is providedadvantageously in the preferred embodiment in order to removepolarization of the light because the beam splitter 408 is sensitive topolarized light, and most LEDs transmit some percentage of polarizedlight. The light then passes to the beam splitter 408 where it isdivided. The beam splitter 408 is preferably coated with a materialwhich is partially reflective to light of the wavelength of the LEDs ofinterest in the LED network 402. Advantageously, the beam splitter 408reflects approximately one-half of the light 414 and directs it to thefirst photodetector 404. The remainder of the light passes through thebeam splitter 408 and through the filter 410 and is received by thesecond photodetector 406. The oximeter system 412 receives the intensityreading from the first and second photodetectors 404, 406 and utilizesthe relative intensities from the first and second photodetectors 404,406 to determine the centroid of the emission wavelength for the LEDs402, as further explained below.

As is well understood in the art, obtaining a beam splitter to preciselydivide the light by 50 percent would be costly to construct. However, itis not necessary to obtain a 50 percent split of the light becauseimprecision can be accommodated with calibration. In an embodiment whereno second filter 411 is provided, the system can be calibrated byactivating the infrared LED. This is possible because the first filter410 is transparent to the infrared wavelength, and thus, eachphotodetector 404, 406 senses the same signal. In such an embodiment,the intensity outputs from the first and second photodetectors 404, 406can be compared and equalized through calibration constants duringrun-time. This compensates for imprecision in the photodetectors, beamsplitter 408 and diffuser 407.

In an embodiment where the infrared is not used to calibrate, thephotodetectors 404, 406, the beam splitter 408 and the diffuser 407 canbe calibrated prior to delivery with a passive or active coding element415 for each device. It should be understood that the box 415 representsone or more coding elements. It should also be understood that a singlecoding element could be used for all of the optical devices within thebox 515. Preferably, the elements provided for calibration (those withinthe box in dotted lines labelled 515) in this embodiment are positionedin a reusable portion of the probe such that the increased expense isnot too significant.

The filter 410 may also have imprecision due to temperature sensitivityand imprecision of manufacturing process. Therefore, in order tocalibrate for imprecision with respect to the filter 410 (preferably ashot glass) due to shift in temperature, a temperature detector 405 isprovided in a preferred embodiment. Because temperature sensitivity inshot glass filters are well known, by detecting the temperature, theshift in filter characteristics can also be determined. With respect tothe imprecision in manufacturing, a passive or active coding element 415can be provided on the probe to provide information about the variationfrom a selected (ideal) filter characteristic (transition band forfilter).

Another preferred embodiment utilizing a filter configuration isdepicted in FIG. 9B. FIG. 9B depicts a sensor having a transmission LEDnetwork 420, a diffuser 421, a first photodetector 422, and a secondphotodetector 424. As in FIG. 9A, a third photodetector 431 is depictedrepresenting the photodetector used for oximetry measurements. The firstand second photodetectors 422, 424 are positioned at the interiorperiphery of an integrating optical sphere 426, or the like. As can beseen in FIG. 9B, the integrating optical sphere 426 has an aperture 428through which light 429 from the LED network 420 is directed formonitoring and for wavelength determination. The light which enters theaperture is reflected about the interior of the optical sphere 426,without significant absorption. Advantageously, the interior of theintegrating optical sphere is reflective to the wavelengths of the lightfrom the LED network 420. In addition, the interior of the integratingoptical sphere 426 scatters the light. Advantageously, the first andsecond photodetectors 422, 424 are spaced laterally across theintegrating optical sphere, with the aperture 428 positionedequidistance between the first and second photodetectors 422, 424. Inthis manner, each of the first and second photodetectors 422, 424receive substantially the same amount of light originating from the LEDnetwork 420.

As with the embodiment of FIG. 9A, the second photodetector 424 has anassociated low pass optical filter 430, through which the light incidenton the second photodetector 424 passes prior to reaching the secondphotodetector 424. Accordingly, like the embodiment of FIG. 9A, thesecond photodetector 424 in FIG. 9B receives light attenuated by thefilter 430, and, the first photodetector 422 receives light unattenuatedby the filter 430.

As with the embodiment of FIG. 9A, as is well understood in the art,obtaining an integrating optical sphere precisely integrate the lightwould be costly to construct. However, again, it is not necessary toobtain a perfect integrating sphere because imprecision in the sphere(as well as in other elements) can be accommodated with calibration. Forinstance, the system of FIG. 9B can be calibrated by activating theinfrared LED if no infrared filter (corresponding to the filter 411 inFIG. 9A) is used. This is possible because the filter 430 is transparentto the infrared wavelength, and thus, each photodetector 422, 424 sensesunfiltered signal (which ideally would be the same). In such anembodiment, the intensity outputs from the first and secondphotodetectors 422, 424 can be compared and equalized throughcalibration constants during run-time. This compensates for imprecisionin the photodetectors, optical sphere, and diffuser.

As with the embodiment of FIG. 9A, if the infrared is not used tocalibrate, the photodetectors 422, 424, the optical sphere 426, and thediffuser 421 can be calibrated prior to delivery with passive or activecoding element(s) 432 for each device.

As with the embodiment of FIG. 9A, the filter 430 may have imprecisiondue to temperature sensitivity and imprecision due to manufacturing.Therefore, in order to calibrate for imprecision with respect to thefilter 430 (preferably a shot glass) due to shift in temperature andmanufacturing tolerances, a temperature detector 425 is provided in apreferred embodiment, as with the embodiment of FIG. 9A. With respect tothe imprecision in manufacturing, a passive or active coding element 432can be provided on the probe to provide information about the variationfrom a selected (ideal) filter characteristic (transition band forfilter).

It should also be understood, that in one embodiment, a single memoryelement or other passive or active element (415, 432) could be providedwith enough identification capability to provide characteristicinformation for each of the diffuser, the photodetectors, filters, andthe beam splitter (or optical sphere). For instance, a memory device ortransistor network could be provided with several bits of informationfor device.

In the present embodiment, with red (e.g., 640-680 nm) and infrared(e.g., 900-940 nm) LEDs in the LED networks 402, 420 of FIGS. 9A and 9B,the wavelength of the red LED is the most critical for blood oximetry.Accordingly, accurate determination of the centroid operating wavelengthof the red LED in the LED networks 402, 420 is desired. In this case,the filters 410, 430 advantageously are selected to partially attenuatelight in the red wavelength range, and pass light in the infrared rangeunattenuated.

The principle by which the sensors of FIGS. 9A and 9B can be used toidentify the wavelength of the LEDs for those sensors is now described.As well understood in the art, LEDs for use in blood oximetry and thelike have an emission characteristic similar to the emission curvedepicted with the curve 440 of FIG. 10A. As depicted in FIG. 10A, theideal LED has a centroid wavelength at λ₀ (e.g., 660 nm). However, aswell understood, the actual centroid wavelength for a batch of LEDs witha target centroid wavelength of λ₀ differs due to manufacturingtolerances. For instance, the emission curve may be shifted to the rightas in the dotted emission curve 440A depicted in FIG. 10A. The actualcentroid wavelength is significant in accurate oximetry measurements.

The filters 410, 430 preferably have a response as depicted by the curve450 in FIG. 10B. With a filter chosen with the middle of its transitionband selected at the target centroid wavelength, λ₀, the filtertransition band advantageously extends from a lower anticipatedwavelength λ₁ to an upper anticipated wavelength λ₂. The range (λ₁-λ₂)preferably encompasses the anticipated variance in wavelengths for LEDsdue to manufacturing tolerances. In other words, the manufacturingtolerance range for LEDs manufactured to have a target wavelength of λ₀,should not extend beyond the upper or lower bounds of the filtertransition band.

For LEDs having a centroid wavelength in the area of the transition bandof the filter, a ratio of the overall intensity detected from a sensorLED without filtering to the intensity of the same sensor LED detectedwith filtering provides useful information, as further explained.

FIG. 10C is illustrative of the ratio for an LED having a wavelengthjust above than the target wavelength λ₀. The LED emission withoutfiltering is represented by the LED emission curve 440A. The emissionwith filtering is depicted by the filtered emission curve 441. Thefiltered emission curve 441 represents the filter response multiplied bythe LED emission without filtering as well understood for filteredemission. The significant ratio is the ratio of the area under thefiltered LED emission curve 441 (illustrated with cross hatching) to thearea of under the unfiltered LED emission curve 440A. It will beunderstood that this ratio will vary from 0-1, for LEDs with a centroidin the range λ₁-λ₂, and assuming the same filter response.

This ratio of the two areas can be determined from the ratio ofintensities received from the photodetectors 404, 406 or 422, 424 asfollows: Let the normalized intensity of the unfiltered light I_(L)(λ)and the intensity of the filtered light, I_(f)(λ) be represented by thefollowing equations.

${I_{L}(\lambda)} = \left\lbrack \frac{1}{1 + \left( {\lambda - I_{\lambda_{0F}}} \right)^{2}} \right\rbrack^{2}$${I_{f}(\lambda)} = \left\lbrack \frac{1}{1 + ^{- {({\lambda - F_{\lambda_{0F}}})}^{2}}} \right\rbrack^{2}$

The energy of the unfiltered light as received by the photodetector 404,422 can be expressed as the integral over the range of wavelengths ofthe LED emission as follows:

E(λ₂,λ₁)_((no filter))=∫_(λ) ₁ ^(λ)2I _(L)(λ)P(λ)dλ  (31)

where I_(L)(λ) is the LED emission vs. wavelength (λ) and P(λ) is thephotodiode response vs. wavelength (λ).

For simplicity, where the photodiode response is “1” (P(λ)=1) in therange of interest (λ₁-λ₂) (in other words, the light emitted from theLED falls within the range of the LED), the signal of the firstphotodetector 404, 422 (no filter) will be as follows:

E(λ₂,λ₁)_((no filter))=∫_(λ) ₁ ^(λ)2I _(L)(λ)(λ)dλ  (32)

Similarly, the energy of the light received by the second photodetector406, 424 which has passed through the filter 410, 430 can be expressedas follows:

E(λ₂,λ₁)_((with filter))=∫_(λ) ₁ ^(λ)2I _(L)(λ)(λ)dλ  (33)

If all LEDs for a batch of sensors have the same peak emission andbandwidth in the area of interest (λ₁-λ₂), and can be represented by thesame equation (30) except for a multiplicative constant I₀, then anormalized ratio of the energies can be defined as follows:

$\begin{matrix}{\begin{matrix}{E_{({norm})} = \frac{{E\left( {\lambda_{2},\lambda_{1}} \right)}_{({{with}\mspace{20mu} {filter}})}}{{E\left( {\lambda_{2},\lambda_{1}} \right)}_{({{no}\mspace{14mu} {filter}})}}} \\{= \frac{I_{0}{\int_{\lambda_{1}}^{\lambda_{2}}{{F(\lambda)}{I_{L}(\lambda)}{\lambda}}}}{I_{0}{\int_{\lambda_{1}}^{\lambda_{2}}{{I_{L}(\lambda)}{\lambda}}}}}\end{matrix}\begin{matrix}{{E_{({norm})}(\lambda)} = \frac{I_{0}{\int_{\lambda_{1}}^{\lambda_{2}}{{F(\lambda)}{I_{L}(\lambda)}{(\lambda)}}}}{I_{0}{\int_{\lambda_{1}}^{\lambda_{2}}{{I_{L}(\lambda)}{\lambda}}}}} \\{= \frac{I_{0}{\int_{\lambda_{1}}^{\lambda_{2}}{{F(\lambda)}{I_{L}(\lambda)}{\lambda}}}}{constant}}\end{matrix}} & (34)\end{matrix}$

The generalized ratio of equation (34) is a ratio of the entire area ofthe LED emission attenuated by filtering (designated with cross-hatchingin FIG. 10C) to the area under the entire LED emission curve.

The function E_(norm) is single valued and monotonic in the area (λ₁-λ₂)and depends only on the centroid wavelength shift of the LED withrespect to the center of the transition band, λ₀, of the filter.

Accordingly, for a filter with a center of the transition band at λ₀,the ratio of the energy detected by second photodetector (filterpresent) to the energy detected by the first photodetector (filter notpresent) in the wavelength range (λ₁-λ₂), will be a value between 0and 1. The precise ratio depends upon the centroid wavelength for theLED under test. As can be seen from FIG. 10C, as the centroid wavelengthincreases toward λ₂, the ratio approaches “1”, and as the centroidwavelength approaches λ₁, the ratio approaches “0”. This relationship isdepicted in FIG. 10D for λ₁=˜610 nm and λ₂=˜710 nm.

In use, a ratio can be calculated to corresponds to each possible LEDwavelength in the range (λ₁-λ₂). For instance, a test batch of LEDsrepresenting the range of wavelengths (λ₁-λ₂) can be used to obtaincorresponding ratios of the intensity of filtered light to unfilteredlight. An accurate wavelength detection device, such as a monochrometer,can be used to measure the centroid wavelength for each tested LED. Thecentroid wavelength can be stored for each tested LED in associationwith the measured ratio for each tested LED. This leads to a normalizedphotodiode response, which can be referenced to obtain the wavelength ofan LED having an unknown wavelength in the wavelength range (λ₁-λ₂).

In other words, for any LED having a centroid wavelength in the range(λ₁-λ₂), with a sensor as depicted in FIGS. 9A and 9B, the wavelength ofthe LED for the sensor can be determined by taking the ratio of theintensities of the second and first photodetectors, and using the ratioto reference the normalized photodiode response to find the wavelength.In the present embodiment, this is accomplished with a look-up tablestored in a memory for the oximeter system. The look-up table stores theratio values corresponding to associated wavelength values.

Accordingly, with the sensor embodiments of FIGS. 9A and 9B, theoximeter simply continually initiates measurements for calibrationpurposes. The oximeter, using the method described above, calculates theratio between the two intensities (filtered and unfiltered) and obtainsthe respective wavelength for the sensor. This is for testing purposes.Accordingly, the LEDs or shot glass purchased advantageously shouldproduce a ration less than 1 and greater than 0, otherwise the LEDwavelength will be undeterminable. In case the ratio equals 1 or zero,the system should either not operate or use a calibration equation thatis closest to the extreme (e.g., for ratio=0, assume wavelength is 630nm and for a ratio=1, assume wavelength is 670 nm in the presentembodiment).

As mentioned above, knowledge about the precise wavelength of the redLED in an oximeter probe is generally more critical than knowledge ofthe precise wavelength of the infrared LED. Accordingly, the filters ofthe sensors of FIGS. 9A and 9B are chosen with the center of theirtransition band, λ₀, in the red wavelength range. As seen from thefilter response curve of FIG. 10B, if the center of the transition bandis in the red range, the infrared light will not be attenuated by thefilter.

Examples of preferable filter responses are depicted in FIG. 11. FIG. 11depicts the response curve for three filters, adequate for the presentinvention, depending upon the expected wavelengths. A first filter hasthe center of its transition band at 645 nm, a second filter has thecenter of its transition band at 665 nm and a third filter has thecenter of its transition band at 695 nm. Other filters are alsoappropriate depending upon the target centroid wavelength.

However, it should be understood that the principle explained abovecould also be used for the infrared LED, if the filters are chosen withthe center of their transition band at λ₀ selected at the anticipated ortarget infrared wavelength (e.g., 905 nm). In addition, the secondfilter 411 (FIG. 9A) can be provided as a filter, with the center of itstransition band selected at the anticipated or target infraredwavelength in order to calibrate the infrared LED as well. In otherwords, the second, filter 411 would pass red wavelengths (would betransparent to the red LED light) and would have its transition bandcentered around 900 or 905 nm. Such a filter is depicted in FIG. 11A.

The wavelength detection described above could also be implemented witha sensor having only one photodetector, and a removable filter. Theoperator would initiate an intensity measurement as prompted by theoximeter without the filter. Then, the operator would place the filterin the light path between the LED and the photodetector, and initiate asecond reading. The ratio of the second reading to the first readingprovides the ratio I_(norm), which is used to reference the operatingwavelength.

Probe Examples

FIGS. 12-14 illustrate three different of probes used in medicalmonitoring of patients.

FIG. 12 depicts a wrap-around type probe 500 with an associatedconnector 502 coupled to a cable 504 which couples to an oximeter system(not shown in FIG. 12). FIG. 12A depicts the bottom of the connector502. FIG. 12B depicts a bottom view of the wrap-around probe of FIG. 12,and FIG. 12C depicts a side view of the wrap-around probe of FIG. 12.The wrap around probe 500 has an LED emitter 506, a photodetector 508 atthe end of a cavity 509, a flexible circuit 510, and friction electricalconnection fingers 512. The probe 500 also has a connection port 519. Inone embodiment, where the probe would be used for the calibratable probeof FIG. 9A, the wrap-around probe would also have a light-tunnel 514(FIG. 12B) to channel some of the light from the emitter 506 to theconnector 502. In such an embodiment, all of the probe calibrationelements marked in the dashed line 515, 515A in FIGS. 9A and 9B arepositioned in a cavity 516 (FIG. 12A) which receives the, lightchanneled through the light tunnel 514 and coupled to the connector 502via an aperture 518 at the end of the light tunnel 514. As seen in FIG.12A, electrical friction connectors 520 on the connector are configuredto couple with the electrical connectors 512 of the wrap-around probe500. The flexible circuit connects the emitters 506 and the detector 508to the connection fingers 512.

In use, the wrap-around probe is placed on the digit of a patient, andthe photodetector 508 is positioned opposite the emitter 506 so as toreceive light from the emitter 506 attenuated by transmission through afleshy medium.

FIG. 13 depicts another embodiment of a wrap-around probe 530 formedical monitoring of infants. The probe has a first flexible portion532 configured to be wrapped about the digit of a neonate attached tothe first flexible portion 532 is a second flexible member carryingemitters 534 (LEDs) and photodetector 536. In one embodiment where thecalibration probe of FIG. 9A is implemented with the probe of FIG. 13, afiber optic 538 is provided to carry part of the light from the emitter534 to the connector port 540 of the probe 530. In this manner, the sameconnector 502 having a photodetector can be utilized with the infantstyle probe of FIG. 13. Alternatively, a light channel or tunnel couldbe used instead of the fiber optic to carry a portion of the light fromthe emitter 534 to the connector port 540. The same connector 542 isused for the neonatal probe 530. Accordingly, as with the embodiment ofFIG. 12, all of the calibration elements within the dotted box 515, 515Aof FIGS. 9A and 9B are positioned within the connector 502.

FIG. 14 depicts yet another probe for use in medical monitoring. Theprobe of FIG. 14 comprises a clip-on probe 550 which couples via a cable552 to a connector port 554 which is the same as the connector port 540of FIG. 13 and the connector port 519 of FIG. 12. The clip-on probecarries emitters 556 and a photodetector 558. With this embodiment, somelight from the emitters 556 enters a fiber optic 560 which channelslight to the connector port 554 as in the embodiment of FIG. 13. Again,the probe calibrations elements within the same connector 502 arepreferably contained within the connector 502 which is advantageouslythe same as the connector for the embodiments of FIGS. 12 and 13.

FIGS. 15-15D depict yet another embodiment of a wrap-around probe 600comprising a flexible wrap portion 602 with an associated connector 604coupled to a cable 506 which couples to an oximeter system (not shown inFIG. 15). FIG. 15 depicts a perspective view of the entire probe 600.FIG. 15A depicts the underside of the connector 604. FIG. 15C depicts atop view of the wrap portion 602 and FIG. 15D depicts a bottom view ofthe wrap portion 602. The connector 604 has two portions: an emitterportion 610 and a connection portion 612. The emitter portion 610advantageously contains the emitters (such as LEDs) for the selectedwavelengths. This emitter portion 610 can be reused for a period oftime, preferably weeks to months, thereby allowing for further reducedcost of the wrap-around portion 602 which is disposable after each use.In other words, emitters need not be provided for each wrap portion 602.Yet, the emitter portion 610 is removably coupled to the connectionportion 612 of the connector 604, allowing the connection portion 612 tobe reusable for a much longer period of time.

In this embodiment, the wrap portion 602 is flexible and disposableafter each use with a very low cost. The wrap portion has a flexiblelayer 626 made from polymer or other flexible materials and has aconnector port 614 on the flexible layer 626. The connector port 614 haselectrical finger friction connectors 616 which are adapted to couple toelectrical finger friction connectors 620 (FIG. 15A) on the bottom ofthe connection portion 612 of the connector 604. The electrical fingerfriction connectors 616 for the wrap portion 602 couple to a flexiblecircuit 618 which connects to a detector 622 which is shielded (notshown) for the detector 622. Two of the connections couple to thedetector 622 and the third is for the shield which is preferably aconventional Faraday shield to protect the detector from electromagneticinterference and the like.

The wrap around probe 600 has an aperture 624 that provides a window forthe transmission of light energy from the emitters in the emitterportion 610. The emitters are positioned to transmit light through anaperture 628 (FIG. 15A) in the emitter portion 610 which is configuredto match with the aperture 624 in the wrap portion 602 when theconnector 604 is positioned in the connection port 614. Thus, the lighttransmits from the emitters in the emitter portion 610 through theaperture 628 in the emitter portion 610 and through the aperture 624 inthe wrap portion 602 when the connector 604 is inserted into theconnector port 614 and the emitters are activated.

In use, the wrap portion 602 is wrapped around a digit of the patient(e.g., a finger) and the detector 622 is positioned to receive lighttransmitted through the aperture 624 and through at least a portion ofthe digit. For instance, the wrap portion 602 can be wrapped around afinger in a manner that the detector 622 is opposite the aperture 624from which light energy is transmitted.

In one embodiment, the probe 600 is used for the calibratable probe ofFIGS. 9A and 9B. In this embodiment, the connection portion 612 has theelements in the dotted boxes 515 and 515A of FIGS. 9A and 9B positionedin the connection portion 612. In this manner, the calibration elementsare reusable, yet work with the LEDS in the emitter portion 610 to forma calibratable embodiment. In such an embodiment, the emitters arepositioned in the emitter portion 610 such that the majority of thelight energy transmits through the aperture 628 and that some lightenergy transmits to a light aperture 620 in the end of the connectionportion 612 (FIG. 15B). The connection portion 612 contains thecalibration elements depicted in the boxes 515 and 515A (FIGS. 9A AND9B) housed in the connection portion 612.

FIG. 15B depicts an end view of the connection portion 612 depicting thelight channel 620 and two electrical connector 613A, 613B which provideconnections for LEDs (red and infrared connected back-to-back in thepresent embodiment) in the emitter portion.

It will be understood that the apparatus and method of the presentinvention may be employed in any circumstance where a measurement oftransmitted or reflected energy is required, including but not limitedto measurements taken on a finger, an earlobe, or a lip. Thus, there arenumerous other embodiments which will be obvious to one skilled in theart. Furthermore, the apparatus and method of the present invention maybe employed for any LED application that is wavelength sensitive. Thepresent invention may thus be embodied in other specific forms withoutdeparting from its spirit or essential, characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the following appended claims. All changes which come within themeaning and range of equivalency of these claims are to be embracedwithin their scope.

1. A method to obtain physiological data relating to a physiologicalparameter without knowing precise operating wavelengths of one or morelight emitting devices in an oximeter sensor, the method comprising:driving a first light emitting device with a first drive current togenerate a first light energy having a first wavelength; transmittingthe first light energy through a medium under test to result in a firstattenuated light energy; driving the first light emitting device with asecond drive current to generate a second light energy having a secondwavelength, wherein the second drive current has a different currentlevel than the first drive current, the first wavelength and the secondwavelength fall within a first predetermined range of operatingwavelengths, and a difference between the first wavelength and thesecond wavelength is approximately equal to a preselected wavelengthshift; transmitting the second light energy through the medium undertest to result in a second attenuated light energy; driving a secondlight emitting device with a third drive current to generate a thirdlight energy having a third wavelength, wherein the third wavelengthfalls within a second predetermined range of operating wavelengths andis distinct from the first wavelength and the second wavelength;transmitting the third light energy through the medium under test toresult in a third attenuated light energy; and calculating thephysiological parameter based on the first attenuated light energy, thesecond attenuated light energy, the third attenuated light energy, andthe preselected wavelength shift.
 2. The method of claim 1, wherein thefirst drive current, the second drive current, and the third drivecurrent have substantially fixed current levels and are providedsequentially by a driver circuit in an oximeter monitor to the oximetersensor.
 3. The method of claim 2, wherein the first attenuated lightenergy, the second attenuated light energy, and the third attenuatedlight energy are provided to a receiving and conditioning circuit in theoximeter monitor for analysis.
 4. The method of claim 3, wherein theoximeter monitor further comprises a controller circuit and a displaycircuit to display the physiological parameter.
 5. The method of claim1, wherein the physiological parameter is determined by one or moreconstituents of blood in the medium under test and each of theconstituents of blood has a substantially constant extinctioncoefficient or a substantially linear extinction coefficient within thefirst predetermined range of operating wavelengths.
 6. The method ofclaim 5, wherein each of the constituents of blood has a substantiallyconstant extinction coefficient within the second predetermined range ofoperating wavelengths.
 7. The method of claim 1, wherein thephysiological parameter is oxygen saturation of blood in the mediumunder test, the oxygen saturation of blood is determined byoxyhemoglogin and reduced hemoglobin, the oxyhemoglobin has asubstantially constant extinction coefficient while the reducedhemoglobin has a substantially linear extinction coefficient within thefirst predetermined range of operating wavelengths, and theoxyhemoglobin and the reduced hemoglobin have substantially constantextinction coefficients within the second predetermined range ofoperating wavelengths.
 8. The method of claim 1, wherein the first lightemitting device comprises a red light emitting diode, and the firstpredetermined range of operating wavelengths is approximately 665 nm to690 nm.
 9. The method of claim 1, wherein the first light emittingdevice comprises a tuning element coupled in parallel with a lightemitting diode, and the tuning element is calibrated during amanufacturing process such that the first light emitting deviceapproximately exhibits the preselected wavelength shift in response to apredefined change in drive current.
 10. The method of claim 1, whereinthe second light emitting device comprises an infrared light emittingdiode and the second predetermined range of operating wavelengths isapproximately 850 nm to 920 nm.
 11. The method of claim 1, whereinlevels of the first drive current, the second drive current, and thethird drive current are preset in an oximeter monitor that is attachedto the oximeter sensor.
 12. The method of claim 1, wherein the oximetersensor is attached to a finger or an earlobe for non-invasive monitoringof the physiological parameter.
 13. The method of claim 1, wherein theoximeter sensor is detachable from an oximeter system for replacement ordisinfection.
 14. The method of claim 1, wherein the preselectedwavelength shift is indicated by an information element in the oximetersensor.
 15. An oximeter system comprising: a detachable sensorcomprising: a first light emitting device configured to receive two ormore different drive levels and to generate corresponding light signalshaving different respective operating wavelengths; and a photodetectorconfigured to detect intensities of respective attenuated light signalsproduced by transmitting the light signals through human tissue carryingblood; and a monitor configured to provide the different drive levels tothe first light emitting device and to receive the detected intensitiesfrom the photodetector, wherein the monitor determines a physiologicalmeasurement based at least in part on the detected intensities andwithout knowing the operating wavelengths of the first light emittingdevice associated with the detected intensities.
 16. The oximeter systemof claim 15, wherein the physiological measurement is calculated usingthe detected intensities and an empirically generated data set ofmeasurements with respect to wavelengths of the first light emittingdevice.
 17. The oximeter system of claim 15, wherein the different drivelevels correspond to driving signals having different current levels.18. The oximeter system of claim 15, wherein the different drive levelscorrespond to driving signals having different voltage levels.
 19. Theoximeter system of claim 15, wherein the physiological measurement isdependent on at least one blood constituent and the monitor drives thefirst light emitting device such that the operating wavelengths arewithin a predefined range associated with a substantially constant or asubstantially linearly changing extinction coefficient of the bloodconstituent.
 20. The oximeter system of claim 15, further comprising anindicator configured to provide information relating to an amount ofchange in the drive levels of the first light emitting device toeffectuate a desired shift in operating wavelength of the first lightemitting device or an amount of wavelength shift in response to apredetermined change in the drive levels of the first light emittingdevice.
 21. The oximeter system of claim 20, wherein the indicatorcomprises a resistor or a memory device on the detachable sensor or on asensor connector between the detachable sensor and the monitor.
 22. Theoximeter system of claim 15, wherein the first light emitting devicecomprises a light emitting diode and a slope adjusting resistorconfigured to cause a preselected shift in wavelength in the lightemitting diode in response to a preselected change in drive level to thefirst light emitting device.
 23. The oximeter system of claim 22,wherein the slope adjusting resistor is a semiconductor substrateresistor that is laser trimmed to produce a desired value duringcalibration of the detachable sensor.